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The present invention relates to a process for the preparation of synthesis gas (xe2x80x9csyngasxe2x80x9d), i.e., a mixture of carbon monoxide and hydrogen, from natural gas. More particularly, the present invention relates to controlling the exit stream composition of a syngas reactor by controlling the feed hydrocarbon composition.
Large quantities of methane, the main component of natural gas, are available in many areas of the world. However, a significant portion of that natural gas is situated in areas that are geographically remote from population and industrial centers (xe2x80x9cstranded gasxe2x80x9d). The costs of compression, transportation, and storage often makes the use of stranded gas economically unattractive. Consequently, the stranded natural gas is often flared. Flaring not only wastes the energy content and any possible economic value the natural gas may have but may also create environmental concerns.
To improve the economics of natural gas transportation and utilization, much research has focused on using the methane component of natural gas as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to higher hydrocarbons is typically carried out in two steps. In the first step, methane is reacted to produce carbon monoxide and hydrogen (i.e., synthesis gas or xe2x80x9csyngasxe2x80x9d). In a second step, the syngas is converted to higher hydrocarbon products by processes such as Fischer-Tropsch synthesis. For example, fuels with boiling points in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the syngas. In addition, syngas may be used for the manufacture of ammonia, hydrogen, methanol, and other chemicals. Less traditional uses of syngas continue to be developed and have increased in importance in recent years, such as in the production of acetic acid and acetic anhydride manufacture. Among the promising new developments in syngas chemistry are routes to ethylene.
There are currently three primary methods for converting methane to syngas. Those methods include: steam reforming (the most widespread), dry reforming (also called CO2 reforming), and partial oxidation. Steam reforming, dry reforming, and partial oxidation ideally proceed according to the following reactions respectively:
CH4+H2O+heatxe2x86x92CO+3H2xe2x80x83xe2x80x83(1) 
CH4+CO2+heatxe2x86x922CO+2H2xe2x80x83xe2x80x83(2) 
CH4+xc2xdO2xe2x86x92CO+2H2+heatxe2x80x83xe2x80x83(3) 
For a general discussion of steam reforming, dry (or CO2) reforming, and partial oxidation, please refer to HAROLD GUNARDSON, Industrial Gases in Petrochemical Processing 41-80 (1998), the contents of which are incorporated herein by reference.
Although a theoretical H2:CO ratio can be calculated for any given reaction, relative amounts of hydrogen and carbon in a syngas product stream depend on many factors including the type of reaction, the process technology, the feedstock composition, and the reactor operating conditions. The theoretical ratio of hydrogen to carbon monoxide in the reactant stream of reactions 1, 2, and 3 can easily be calculated as 3:1, 2:2 (i.e., 1:1), and 2:1. The actual ratio of hydrogen to carbon monoxide in syngas product streams can range as low as 0.6 with CO2 reforming of natural gas or partial oxidation of petroleum coke to as high as 6.5 with steam methane reforming. In addition, it has been noticed in GUNARDSON on pages 68-71 the actual molar ratio of H2:CO in the product stream can vary depending upon the feedstock used.
There are many processes, such as the production of methanol, in which an H2:CO molar ratio of about 2:1 is desired. There are also processes in which a molar ratio of hydrogen and carbon monoxide of less than 2:1 is preferable. One such process is hydroformylation, which is the addition of one molecule of carbon monoxide and one molecule of hydrogen to an olefin to make an aldehyde. The following reaction illustrates one of the simplest examples of hydroformylation:
C2H4+CO+H2xe2x86x92CH3CH2CHOxe2x80x83xe2x80x83(4) 
Hydroformylation is, inter alia, an intermediate step in both methyl methacrylate synthesis and the oxo process to produce alcohols. Additionally, there may be other processes in which an H2:CO ratio of between 2:1 and about 1:1 is desirable.
As noted above, one method of producing syngas with a molar ratio of hydrogen to carbon monoxide of between about 2:1 and about 1:1 is by the partial oxidation of methane followed by the CO2 reforming of methane. Unfortunately, CO2 reforming is endothermic and requires external heating to drive the reaction, which increases the capital cost of CO2 reforming. In addition, this scheme of partial oxidation followed by CO2 reforming requires two reactors thereby also increasing the capital cost. Thus, in many situations, partial oxidation followed by CO2 reforming may be economically or physically (or both) unfeasible or undesirable.
There is, therefore, a need for a less capital intensive process in which the H2:CO molar ratio in the product stream can be varied and controlled between about 2:1 and about 1:1
The present invention provides a method for controlling the H2:CO molar ratio between about 2:1 and about 1:1 in a syngas product stream by controlling the feed hydrocarbon composition.
An embodiment of the present method generally includes predetermining a desired syngas product stream H2:CO molar ratio, selecting a hydrocarbon with an actual natural H2:CO molar ratio greater than the desired molar ratio, selecting a hydrocarbon with an actual natural H2:CO molar ratio less than the desired molar ratio, mixing the two hydrocarbons on-line such that the actual natural H2:CO molar ratio of the mixture is equal to the desired molar ratio, and net catalytically partially oxidizing the mixture to produce syngas with the desired H2:CO molar ratio.
It is also possible to control and vary the product stream composition by controlling and varying the feed stream composition.